At present, Orthogonal Frequency Division Multiplexing (OFDM) is widely employed as a transmission method in various digital communications standards (IEEE802.11a, etc.) for digital terrestrial broadcasting and the like. OFDM is a transmission method in which sub-carriers orthogonal to each other are used to frequency-multiplex a plurality of narrow-band digital modulated signals, with a high level of efficiency of use of frequency. Also in OFDM, each symbol duration includes a useful symbol duration and a guard interval. In each symbol, a portion of a signal in the useful symbol duration is copied into the guard interval. Thereby, OFDM is less affected by Inter-Symbol Interference (ISI) between symbols caused by multipath interference, and is highly resistant to multi path interference.
On the ether hand, OFDM signals have a longer length (duration) of a symbol than that of broad-band digital modulated signals. Therefore, OFDM is less resistant to changes in amplitude and phase in a multipath fading environment. Also, in a multipath fading environment during mobile reception, a change in frequency called Doppler spread occurs in addition to changes in amplitude and phase of a received signal caused by delay and dispersion due to multipath. In OFDM, the Doppler spread impairs the orthogonal relationship between individual sub-carriers, leading to interference between sub-carriers, and as a result, accurate demodulation becomes difficult to achieve.
The aforementioned interference between sub-carriers is called Inter-Carrier Interference (ICI). Suppression of a deterioration in reception performance due to ICI is largely desired for mobile communications employing a multi-carrier transmission method.
As an exemplary method for reducing the reception performance deterioration due to ICI, Non-Patent Document 1 described below (hereinafter simply referred to as Document 1) din closes an ICI suppressing method. FIG. 56 is a block diagram showing a configuration of an ICI removing unit 100 included in a conventional OFDM reception apparatus 1100 (not shown) described in Document 1. Note that parts other than the ICI removing unit 1000 of the conventional OFDM reception apparatus 1100 are commonly known and will not be described in detail. The conventional OFDM reception apparatus 1100 subjects a received signal to an FFT process using a Fast Fourier Transform (FFT) unit (not shown) before extracting data included in each sub-carrier. The conventional OFDM reception apparatus 1100 also removes an ICI component from the FFT-processed received signal using the ICI removing unit 1000.
Hereinafter, the ICI removing unit 1000 included in the conventional OFDM reception apparatus 1100 will be described. As shown in FIG. 56, the ICI removing unit 1000 comprises a transmission channel characteristic estimating unit 1001, a temporary equalization unit 1002, is transmission channel characteristic first-differentiation calculating unit 1003, a multiplication unit 1004, a subtraction unit 1005, and delay devices 1011 to 1013.
Here, the FFT-processed received signal is represented by Y[p+1] ([p+1] is a vector value, p is an integer, a number inside the brackets is a symbol number). Y[p+1] Is input to the delay device 1011 and the transmission channel characteristic estimating unit 1001. The delay device 1011 delays Y[p+1] by one symbol to output Y[p]. On the other hand, the transmission channel characteristic estimating unit 1001 uses Y[p+1] to estimate a transmission channel characteristic of each sub-carrier included in Y[p+1] to calculate H[p+1] (H[p+1] is a vector value, p is an integer, and a number inside the brackets is a symbol number). Here, H[p+1] is an estimate of the transmission channel characteristic that represents values indicating distortions in amplitude and phase of Y[p+1] caused by propagation through a transmission channel between a sender and a receiver. The delay device 1012 receives H[p+1] from the transmission channel characteristic estimating unit 1001, and delays H[p+1] by one symbol to output. H[p]. The temporary equalization unit 1002 divides Y[p] input from the delay device 1011 by H[p] input from the delay device 1012 (temporary equalization process) to output a temporary estimate X^[p] (X^[p] is a vector value, p is an integer, and a number inside the brackets is a symbol number) of a transmission signal. Note that the notation of X^[p] is slightly different from that shown in the drawings for the sake of convenience. The delay device 1013 receives H[p] from the delay device 1012, and delays H[p] by one symbol to output H[p−1].
The transmission channel characteristic first-differentiation calculating unit 1003 uses Expression 1 including H[p+1] input from the transmission channel characteristic estimating unit 1001, H[p−1] from the delay device 1013, N that is an FFT size, and Ng that is the product of N and a guard interval ratio, to calculate H′[p] that is the first differentiation of H[p] (H′[p] is a vector value, p is an integer, and a number inside the brackets is a symbol number).
                                          H            ′                    ⁡                      [            p            ]                          =                                            H              ⁡                              [                                  p                  +                  1                                ]                                      -                          H              ⁡                              [                                  p                  -                  1                                ]                                                          2            ⁢                          (                              N                +                                  N                  G                                            )                                                          (        1        )            
In other words, the transmission channel characteristic first-differentiation calculating unit 1033 uses the transmission channel characteristic values H[p−1] and H[p+1] corresponding to symbol numbers before and after the symbol number p, so as to calculate the first differentiation H′[p] of H[p]. The multiplication unit 1004 uses X^[p] input from the temporary equalization unit 1002, H′[p] input from the transmission channel characteristic first-differentiation calculating unit 1003, and a fixed coefficient matrix (FFT leakage matrix) Ξ indicated by Expressions 3 and 4 to calculate an ICI component that is indicated in the second term on the right side of Expression 2.
                                          Y            ^                    ⁡                      [            p            ]                          =                              Y            ⁡                          [              p              ]                                -                      Ξ            ⁢                                                  ⁢                          diag              ⁡                              (                                                      H                    ′                                    ⁡                                      [                    p                    ]                                                  )                                      ⁢                                          X                ~                            ⁡                              [                p                ]                                                                        (        2        )                                Ξ        =                  (                                                    0                                                              ζ                  1                                                            …                                                              ζ                                      N                    -                    1                                                                                                                        ζ                                      -                    1                                                                              0                                            …                                                              ζ                                      N                    -                    2                                                                                                      ⋮                                            ⋮                                            ⋱                                            …                                                                                      ζ                                      1                    -                    N                                                                                                ζ                                      2                    -                    N                                                                              …                                            0                                              )                                    (        3        )                                          ζ          n                =                              -                          1              2                                -                      j                          2              ⁢                                                          ⁢                              tan                ⁡                                  (                                      π                    ⁢                                                                                  ⁢                                          n                      /                      N                                                        )                                                                                        (        4        )            
The subtraction unit 1005 uses Expression 2 in which the ICI component input from the multiplication unit 1004 is subtracted from Y[p] input from the delay device 1011, to calculate a vector Y^[p] that is a received signal excluding the component (Y^[p] is a vector value, p is an integer, and a number inside the brackets is a symbol number). Note that the notation of Y^[p] is slightly different from that shown in the drawings for the sake of convenience.
The aforementioned technique of Document 1 has a feature that H′[p] of the symbol number p that is used to calculate an ICI component (the second term on the right side of Expression 2) included in Y[p] is obtained by a difference operation of the transmission channel characteristic values H[p−1] and H[p+1] of symbol numbers before and after the symbol number p (see Expression 1).    Non-Patent Document 1: Low Complexity Inter-Carrier Interference Compensation for Mobile Reception of DVB-T [9th International OFDM—Workshop 2004, Dresden] (P 72-76, FIG. 4)